It has become common to treat a variety of medical conditions by introducing an implantatile medical device into the alimentary, circulatory, coronary, urological, renal, and other organ systems. For example, coronary vessels via delivery catheters, such as balloon catheters.
In the case of aneurysm treatment, an aneurysm is caused by a weakening of the vessel wall, which causes an invagination of the vessel wall. Blood flow is inhibited at the neck of the aneurysm due to turbulence caused by blood entering and exiting the lumen of the aneurysm. Current medical treatment of aneurysms include the use of metal coils, such as the FDA approved Gugliemi Detachable Coil, inserted into the lumen of the aneurysm. However, this platinum coil is relatively soft and does not provide a complete packing of the aneurysm lumen. It is not uncommon for the aneurysm to re-canalize, enlarge, and even rupture. The heavy metal used in the coils provide the necessary radiographic visualization to ensure that the coils are localized properly and whether, during a subsequent examination, the coils remain in the situs.
However, there are problems associated with using synthetic materials, which include thrombus formation, immune response leading to rejection, and undesired occlusion of the vessel. Therefore, a better material for implantation in any application, such as coronary, vascular, body wall repair, orthopaedic, tissue graft, dermal, and other industries is needed. One such material is a newly discovered biomaterial comprising tissue mucosa, tissue serosa, or tissue submucosa.
Tissue implants in a purified form and derived from collagen-based materials have been manufactured and disclosed in the literature. Cohesive films of high tensile strength have been manufactured using collagen molecules or collagen-based materials. Aldehydes, however, have been generally utilized to cross-link the collagen molecules to produce films having high tensile strengths. With these types of materials, the aldehydes may leech out of the film, e.g. upon hydrolysis. Because such residues are cytotoxic, the films are poor tissue implants.
Other techniques have been developed to produce collagen-based tissue implants while avoiding the problems associated with aldehyde cross-linked collagen molecules. One such technique is illustrated in U.S. Pat. No. 5,141,747 wherein the collagen molecules are cross-linked or coupled at their lysine epsilon amino groups followed by denaturing the coupled, and preferably modified, collagen molecules. The disclosed use of such collagen material is for tympanic membrane repair. While such membranes are disclosed to exhibit good physical properties and to be sterilized by subsequent processing, they are not capable of remodeling or generating cell growth or, in general, of promoting regrowth and healing of damaged or diseased tissue structures.
In general, researchers in the surgical arts have been working for-many years to develop new techniques and materials for use as implants to replace or repair damaged or diseased tissue structures, for example, blood vessels, aneurysms, muscle, ligaments, tendons and the like. It is not uncommon today, for instance, for an orthopedic surgeon to harvest a patellar tendon of autogenous or allogenous origin for use as a replacement for a torn cruciate ligament. The surgical methods for such techniques are known. Further, it has been common for surgeons to use implantable prostheses formed from plastic, metal and/or ceramic material for reconstruction or replacement of physiological structures. Despite their wide use, surgical implanted prostheses present many attendant risks to the patient.
Researchers have also been attempting to develop satisfactory polymer or plastic materials to serve as functional tissue structures and/or other connective tissues, e.g., those involved in hernia and joint dislocation injuries. It has been discovered that it is difficult to provide a tough, durable plastic material which is suitable for long, term connective tissue replacement. The tissues surrounding the plastic material can become infected and difficulties in treating such infections often lead to the failure of the implant or prostheses.
As mentioned above, various collagen-based materials have also been utilized for the above-mentioned tissue replacements; however, these materials either did not exhibit the requisite tensile strength or also had problems with infection and other immunogenic responses, encapsulation, or had other problems. In a related patent, U.S. Pat. No. 5,372,821, it is disclosed that a submucosa collagenous biomaterial may be sterilized by conventional techniques, e.g., aldehyde tanning, propylene oxide, gamma radiation and peracetic acid. No specific processing steps are disclosed except that the submucosa layer is first delaminated from the surrounding tissue prior to sterilization treatment.
Some materials considered desirable are biological materials (biomaterials) from autogenous, allogenous, or xenogeneic (heteroplastic) sources. Biomaterials are desirable as they can be malleable and less likely to be rejected as foreign. One such biomaterial is collagen. Collagen is a protein molecule that comes in many types. For example, collagen Type I constitutes a significant amount of the collagen in the body. Type I is a heterotrimeric molecule, has a helical configuration, and is characterized by a Glycine-X-Y amino acid repeating sequence. Due to its abundance in the human body, collagen is being examined for its uses in medical treatment.
One of the problems associated with biomaterials includes leakage or seepage from the tubular graft. Particularly, a graft made by suturing two ends of a flat sheet together cause holes to extend from the lumenal side to the outside, thus providing small channels for lumenal fluid to seep. A reduced seepage biomaterial construct is desired and would be well-received.
Problems associated with synthetic grafts are well-documented. For example, it is known that the mechanical properties of synthetic grafts degrade over time, as described in Vinard et al, Stability of Performances of Vascular Prostheses Retrospective Study of 22 Cases of Human Implanted Prostheses, Vol. 22(7) Jour. of Biomedical Materials Research pg. 633-648 (July 1988); and Manfredi et al., Vascular Prostheses, vol. 12(3) Emergency Medicine Clinics of North America pg. 657-77 (August 1994). A graft that gets stronger over time is more desirable than one that degrades over time.
One other problem associated with synthetic or biomaterial grafts include the occlusion of the lumen itself. Often times, the graft materials come loose or the layers comprising the graft separate, thus causing material to hang into the lumen. This debris causes thrombogenesis and reduces patency of the graft. If the thrombus were to dislodge, disastrous effects will soon follow.